Technical Field
This invention relates to a method and apparatus for combining two reactive gases while delaying the reaction between such gases as one flows into the other. More particularly, this invention relates to a method and apparatus for introducing reactive gases into an exhaust gas stream in such a manner that reactions between reactive species in such gases are not permitted in harmful locations, but are permitted in non-harmful locations.
Many semiconductor device fabrication processes are carried out under vacuum conditions in order to reduce contamination of the device surface layers being produced and to improve control of the microstructure and chemical homogeneity of any etched or deposited layers. Various reactive gases and vapors are used in this process with the specific vapor phase species dependent upon the desired result of the particular unit operation. Further, since these are production processes, reaction rates must be as high as practical and equilibrium conditions are not achieved in practice, nor do deposition efficiencies approach 100%. As a result, a mixture of reactive vapor species including starting materials and reaction products can be found in the vacuum pump exhaust coming from such processes.
Typically, such vacuum pump exhaust is fed to a common duct system called xe2x80x9chouse exhaustxe2x80x9d or xe2x80x9creactive gas exhaust,xe2x80x9d for transport to an air pollution control device and a catastrophic release containment system, prior to being vented to the atmosphere. Gases, vapors and liquid and solid particulate matter of sufficient buoyancy are drawn through this house exhaust system and vented to the atmosphere by a suction fan which typically creates about 2 to about 5 inches water negative pressure. Gas velocities in the house exhaust system are typically from about 10 to about 30 feet per second under these conditions on average.
Since the vapor stream passing through the xe2x80x9chouse exhaustxe2x80x9d duct system contains exhaust from many processes, and thus a mixture of reactive vapor species, reaction of species entering the system at different points can occur. Exhaust system reactions can be eliminated to a certain extent by segregation of reactive species categories to specific exhaust systems such as xe2x80x9ccorrosive,xe2x80x9d xe2x80x9cvolatile organic compoundxe2x80x9d (VOC), etc., but there is a limit to what can be achieved with this scheme at a practical facilities cost.
Some reaction between species in the house exhaust is therefor unavoidable. The problem then becomes one of controlling the location of the reaction. That is to say minimizing reaction in xe2x80x9charmfulxe2x80x9d locations, and maximizing the fraction of the reaction taking place in xe2x80x9cnon-harmfulxe2x80x9d locations. For example, where species in the xe2x80x9chouse exhaustxe2x80x9d can react with species contained in a particular vacuum pump exhaust being fed to the house exhaust, reaction will take place in the vicinity of the junction of the two streams. Exactly where in the vicinity of the junction of the two streams is the crucial issue. Specifically, xe2x80x9cbackstreamingxe2x80x9d of reactant species from the house exhaust past the junction of the two streams into the vacuum pump exhaust can occur. When such xe2x80x9cback-streamingxe2x80x9d occurs, it can result in reaction product deposition in the vacuum pump exhaust lines under the right conditions. This is a xe2x80x9charmfulxe2x80x9d location since such deposition can plug the vacuum pump exhaust lines to the extent that vacuum pumping speed is reduced from nominal or optimal, with the result that process control is adversely affected.
It is thus of great value to eliminate reactive vapor backstreaming as is accomplished by the invention herein described, and to cause the reaction between the reactive gases to occur in a non-harmful location rather than in a harmful location such as the vacuum pump exhaust line where clogging can occur.
This invention is directed to a method and apparatus for delaying the reaction between a first reactive gas and a second reactive gas as the first reactive gas flows into the second reactive gas. The method comprises the steps of surrounding the first reactive gas with a non-reactive gas to form an insulated first reactive gas and flowing the insulated first reactive gas into said second reactive gas. The method may also comprise the step of sweeping the insulated first reactive gas with a sweep gas to provide it with increased momentum during the flowing step. The apparatus comprises first and second coaxial tubular members, means for flowing a first reactive gas through the first and inner tubular member, means for flowing a non-reactive gas through the annular space between the coaxial tubular members, and a chamber through which a second reactive gas flows. The apparatus may also comprise a third coaxial tube member and means for flowing a sweep gas through the annulus between the second and third tubular members to impart momentum.